This invention relates to a cathode ray tube for projection television, and more particularly relates to such a tube having a interference filter between the display window and the luminescent layer, and also relates to a projection television device incorporating such a tube.
Tubes of this type are described in U.S. Pat. No. 4,634,926 assigned to U.S. Philips Corporation, in which the filter is composed of alternating layers of materials of high and low refractive index. The filter is designed to result in a marked increase in luminous efficiency of the tube in the forward direction, as well as improved chromaticity and contrast. Even further improvements are provided, especially in light gain in the corners of the display screen, by combining such an interference filter with an inwardly curved display window, as provided in U.S. Pat. No. 4,683,398, also assigned to U.S. Philips Corporation.
In these tubes, the interference filter can be characterized as an SWP filter, (short wave pass filter), that is, it has relatively high transmittance at wavelengths below a relatively narrow transition or cut-off region, and relatively high reflectance at higher wavelengths. See, for example, FIG. 6 of U.S. Pat. No. 4,683,398.
While such filters have generally proven to be quite effective for use in the red, green and blue tubes of a three-tube color projection television device, in practice it has been found that an objectionable off-color cast can occur. For example, variations in thickness distribution of the green filter can result in an objectionable bluish cast in the green tube incorporating a Tb-activated green phosphor.
This can be appreciated if it is realized that the emission spectrum of the green phosphor generally includes a blue component peaking at about 480 nm below the cut-off region of the filter. (Again, see FIG. 6 of U.S. Pat. No. 4,683,398, which includes a typical emission spectrum for a Tb-activated green phosphor). Generally, this blue component is not significant, due to its weakness relative to the dominant green component, peaking at about 540 nm, as well as to the greater sensitivity of the human eye to green.
However, failure to adequately control thickness distribution across the display window can lead to insufficient thickness in certain areas, and consequently to a shift of the cut-off region toward lower wavelengths. If this shift results in a partial cut-off of the main, desired component of emission, an objectionable off-color cast may appear in the areas of smaller thickness.
Accordingly, it is an object of the invention to provide a projection television display tube having an interference filter which substantially eliminates the blue component of emission of such a tube having a Tb-activated green phosphor screen.
It is another object of the invention to provide a projection television display tube having an interference filter which reflects emissions occurring at wavelengths both above and below the region of primary color emission.